Hydroxylated derivatives of dimethoxy-1,4-benzoquinone as redox switchable earth-alkaline metal ligands and radical scavengers (original) (raw)

Benzoquinones (BQ) have important functions in many biological processes. In alkaline environments, BQs can be hydroxylated at quinoid ring proton positions. Very little is known about the chemical reaction leading to these structural transformations as well as about the properties of the obtained hydroxyl benzoquinones. We analyzed the behavior of the naturally occurring 2,6-dimethoxy-1,4-benzoquinone under alkaline conditions and show that upon substitution of methoxy-groups, poly-hydroxyl-derivatives (OHBQ) are formed. The emerging compounds with one or several hydroxyl-substituents on single or fused quinone-rings exist in oxidized or reduced states and are very stable under physiological conditions. In comparison with the parent BQs, OHBQs are stronger radical scavengers and redox switchable earth-alkaline metal ligands. Considering that hydroxylated quinones appear as biosynthetic intermediates or as products of enzymatic reactions, and that BQs present in food or administered as drugs can be hydroxylated by enzymatic pathways, highlights their potential importance in biological systems. Q uinones constitute a broad class of biologically active substances (small molecules) involved in vital cellular processes such as respiration and photosynthesis 1-4 . In addition, there is also an increasing number of quinoid compounds produced mainly by plants and fungi, for which antineoplastic or antibiotic features have been described 5,6 . In the respiratory chain, the prime role of coenzymes Q is to mediate the electron transfer between various redox centers and to translocate protons across the inner mitochondrial membrane by turnover of the quinone/quinol (Q/H 2 Q) redox couple. Because of these redox transitions, in cells, coenzymes Q can act as weak radical scavengers 7 and also as a source of superoxide ( N O 2 -) and related oxidants 8 . For quinones in general, the structure of the quinoid core group and its substituents determines their redoxactivity and chemistry which in case of the biologically and pharmacologically important benzoquinones is still not fully resolved. While the mechanistic pathway of electron transfer of quinones in organic (aprotic) media involving two successive one-electron steps seems to be unanimously defined and accepted, in aqueous solutions, quinone electrochemistry is still controversial 9-13 .